The present invention relates to an absorbent article that includes a liquid-permeable outer casing sheet mounted on a first surface of the article, a liquid-impermeable outer casing sheet mounted on a second surface of the article, and an absorbent body enclosed between said two casing sheets, wherein the absorbent body includes a liquid-reception or liquid acquisition space consisting of at least one cavity or at least one region of lower density than an acquisition layer of the absorbent body that adjoins said space essentially in the same plane thereas, and wherein the liquid-acquisition layer includes a material which when wetted increases in size in a direction (z-direction) generally perpendicular to the first surface of the article.
One problem with hitherto known absorbent articles, such as diapers, pants-type diapers or trainers, incontinence guards, sanitary napkins and like articles, that are intended to repeatedly receive and absorb body fluid excreted by a user is that the rate at which liquid is able to penetrate into the article is greatly reduced at each renewed wetting occasion. This problem is particularly apparent in diapers and incontinence guards for children and adults, since the article must be capable of receiving and absorbing a relatively large volume of liquid excreted in the course of only a few seconds. It is therewith not unusual, particularly after a first wetting of the article, that liquid which is not immediately absorbed into the article will instead flow across the surface of said article and run past the edges thereof Such body fluid leakages are, of course, highly undesirable because, among other things, they soil and stain clothes, bed linen and mattresses, sometimes destructively.
The reason why the body fluid acquisition rate decreases with repeated wettings of the article is because the absorbent body of the article becomes temporarily saturated with body fluid within a limited area around the point at which the surface of the article is first met by the body fluid, the so-called primary wetting point. As a rule, the absorbent articles include one or more layers of hydrophilic fibres, for instance cellulose fluff pulp, and often also a powerful absorbent hydrocolloidal material, so-called superabsorbents. Liquid transport in such material is relatively slow, since it is primarily, dependent on capillary forces in those cavities present between fibres and particles in the absorbent body of the article. Liquid is transported within the hydrocolloidal materials by diffusion, which is a slower process than that generated by the capillary forces. Thus, the liquid will remain in the primary wetting region of the article for a relatively long period of time, and is then transported only gradually to surrounding parts of the absorbent body of said article.
In recent years, the problem has been accentuated by the development towards absorbent bodies that have been successively compressed to greater extents with the intention of reducing packaging volume and for transportation, storage and environmental reasons.
It is known to guide the transportation of liquid away from the primary wetting region to those parts of the absorbent body in which absorbent material that has still not been utilized is located, by providing the article with liquid transporting means in the form of compressed patterns, e.g. compressed strips, that disperse the liquid in the longitudinal direction of the article. An article that includes such compressed strips is known from PCT/SE94/00835. Liquid transportation in the article is mainly effected as a result of differences in capillary forces between the compressed strips and surrounding material. Although a certain positive effect is obtained in this way, in the form of a directed flow of liquid in the absorbent body, the rate at which the liquid is transported in the article is much too slow in relation to the rate at which body liquid is discharged onto the article. There is thus a risk that liquid will not be absorbed quickly enough and will instead run along the surface of the article and leak over the edges thereof. This risk is particularly applicable to products that are intended for urine absorption, such as diapers and incontinence guards, which must often deal with large volumes of liquid that are discharged within a relatively short time. Furthermore, heavy compression of an article makes the article stiff and imparts thereto parts that are not-readily flexible and that make the article less pliable and less liable to mould to the shape of the wearer""s body when worn.
The capacity of an absorbent article to receive and retain large volumes of body liquid can also be enhanced by creating different types of liquid-receiving cavities or basins in the article.
U.S. Pat. No. 3,889,679 describes a diaper that has a plurality of circular holes disposed through the absorbent body of the diaper. However, since wetting of a diaper takes place within a limited region of the diaper, the primary wetting region, only those holes that are located nearest this region can be used to initially receive body liquid. These holes quickly fill with liquid, which is gradually drained by the surrounding absorbent material away from the holes by the suction effect generated by the capillary forces between the fibres in the absorbent material. This is a slow process, as before mentioned, and there is a considerable risk that liquid will still be left in the holes when liquid is discharged on the next wetting occasion. The absorbent material located nearest the primary wetting region of the diaper will gradually become saturated with body liquid and then lose essentially all ability to drain liquid from the holes. Another problem with this absorbent body is that it consists of a material which collapses when wetted and therewith essentially lose its three-dimensional structure. Consequently, the cavity space available in the absorbent body for acquisition of liquid is practically non-existent after a first wetting.
U.S. Pat. No. 4,560,372 describes an absorbent body that includes a resilient fibre layer and a layer of hydrophilic fibres that have been compressed, slit and then drawn apart to form openings. The absorbent body also includes 20-60% superabsorbent material. As a result of its high superabsorbent content, the material will swell in both the z-direction and in the xy-direction when wetted, i.e. the material will swell into the openings at the same time as it swells in the thickness direction of said material, wherewith the area of said openings are greatly reduced after wetting.
Swedish Patent Application No. 9304321-4 describes an absorbent body for absorbent articles such as diapers, incontinence guards and sanitary napkins that is provided with a liquid-acquisition part in the form of a drainage well that is located essentially opposite the contemplated primary wetting region of the absorbent body and that extends down into and through a liquid storage part of the absorbent body. The drainage well is in liquid communication with a liquid dispersion layer located beneath a liquid storage layer and has a greater effective mean pore size than the surrounding liquid storage part.
An absorbent body of this kind will function effectively in respect of receiving a first volume of liquid and even subsequent liquid volumes when the time between liquid discharges is sufficient to enable the well to empty of liquid between the liquid discharge occasions. The absorbent body according to Swedish Patent Application No. 9304321-4 is also dependent on capillary forces in draining liquid from the well. Consequently, the liquid-acquisition well is emptied gradually as liquid is transported from the coarser pores in the well to the finer pores in the surrounding absorbent material. There is also the risk that the well will be too small to accommodate large volumes of liquid and therewith be overfilled.
Prior publications WO 87/01914, U.S. Pat. No. 4,333,462, U.S. Pat. No. 4,333,463, U.S. Pat. No. 4.333,464, U.S. Pat. No. 4,413,996, EP 0,124,365, GB 2,156,681, U.S. Pat. No. 4,643,727 and EP 5,528,567 also describe similar products provided with liquid receiving and collecting cavities or basins.
There still remains, however, a significant demand for an absorbent article that can be wetted repeatedly at a high liquid acquisition rate, even with respect to the subsequent wetting occasions.
Our earlier invention described in our copending PCT-application WO96/20670 concerns an absorbent article which includes a li be wetted repeatedly at a high liquid acquisition rate, even with respect to the quid-permeable outer sheet disposed on a first surface of the article, a liquid-impermeable outer sheet disposed on a second surface of the article, and an absorbent body which is enclosed between the two casing sheets and which includes a body liquid receiving space comprising at least one cavity or region of lower density than the density of a part of the absorbent body located adjacent the receiving space and extending generally in the same plane thereas, the article being characterized in that the receiving space is disposed in a storage layer in the absorbent body; and in that parts of the storage layer adjacent the receiving space include a material which, when wetted, increases in volume in a direction (z-direction) generally perpendicular to the first surface of the article, whereby the size of the receiving space also increases in said direction as a result of the article being wetted.
However, the sole requirement that material of the storage layer adjacent the receiving space shall increase in volume in the z-direction does not always guarantee a substantial increase of the volume of the liquid receiving space and does, therefore, not necessarily, under all circumstances, give essentially improved liquid-acquisition properties at repeated wettings.
It is therefore an object of the present invention to achieve an absorbent article of the kind defined in the introduction having substantially improved liquid acquisition properties at repeated wettings.
There has been produced in accordance with the invention an article of the kind defined in the introduction that provides a solution to this problem. The inventive article is mainly characterized in that when wetted, the material present in the acquisition layer exhibits relatively low expansion in a direction (xy-direction) essentially parallel with the first surface of the article, such that the volume of the liquid-acquisition space will increase by at least 100%, preferably at least 200%, even more preferably 400% and most preferably at least 900% when wetted to saturation with a 0.9%-NaCl solution.
Expansion of the material in the xy-direction will not preferably be greater than that which results in a decrease in the area of the liquid-acquisition space in the xy-direction by at most 25%, preferably at most 20% and most preferably at most 10% when wetted.
Expansion of the material in the z-direction will preferably be at least 100%, preferably at least 200%, even more preferably at least 400% and most preferably at least 900%.
A storage layer is preferably disposed in liquid communication with the liquid-acquisition layer on that side of the layer that lies proximal to the liquid-impermeable outer casing sheet. The storage layer will preferably include a material having good liquid retention properties, such as cellulose fibres in combination with superabsorbent material, tissue laminates with superabsorbent material, or absorbent foam material.
According to one preferred embodiment, a dispersion layer is disposed in liquid communication with the absorption layer, either between said layer and the storage layer or between the storage layer and the liquid-impermeable outer casing sheet. The dispersion layer will preferably include a material that has good liquid dispersion properties, such as compressed cellulose pulp, a fibre mat or wadding of synthetic or natural fibres or an open-cell foam material.
There is preferably disposed between the liquid-permeable outer casing sheet and the acquisition layer a liquid transportation layer that includes a material capable of quickly accepting liquid and quickly releasing the liquid to the underlying layer. This transportation layer may be comprised of a lightly-compressed cellulose fluff layer of mechanical, thermomechanical, chemithermomechanical pulp (CTMP) chemical fibres that have been chemically stiffened or cross-linked, a fibre mat or wadding of synthetic or natural fibres or an absorbent foam material.
According to one embodiment of the invention, the liquid-acquisition space of one or more holes or regions of lower density than the density of the surrounding material in the acquisition layer extends/extend through at least a part of the thickness of the acquisition layer.
According to another embodiment, the acquisition layer is comprised of at least two separate material bodies that extend in the form of pillar-like spacer means generally perpendicular between two further material layers in the article, and together with said material layers delimit a coherent acquisition space between the material layers. According to another embodiment, the acquisition space is comprised of at least one channel-like cavity extending in the longitudinal direction of the article.
According to still another embodiment of the invention, the acquisition layer is formed by a web of material that is divided in the longitudinal direction of the web by an undulating curve, wherein the web parts are offset in relation to one another in the plane of the web, at least in the longitudinal direction thereof, such that the web parts define the acquisition space therebetween in the plane of the material web. By undulating curve is meant a curve of optional shape, such as a sinusoidal shape, a saw-toothed shape, a square-wave shape, and so on. The amplitude of the waves and their lengths can vary along the curve. The waves may extend along a straight, curved or wavy line.
A particularly suitable material for use in an inventive article for forming the acquisition layer that surrounds the liquid-acquisition space is one comprised of cellulose fibres of mechanical, thermomechanical or chemithermomechanical (CTMP)pulp and/or chemical pulp fibres that have been chemically stiffened or cross-linked, said fibres being formed into a web having a weight per unit area of 30-2000 g/m2, preferably 50-1500 g/m2 and more preferably 100-1000 g/m2 and compressed to a density of between 0.2-1.2 g/cm3, preferably 0.25-1.0 g/cm3 and most preferably 0.3-0.9 g/cm3. The cellulose fibres may conveniently be comprised of flash-dried fibres that have been dry-formed into a web and incorporated in the article without defibring and fluff-forming. Such a material is described in WO 94/10956. The material may be compressed to a first density of between 0.2-1.2 g/cm3 and thereafter softened mechanically to a density lower than the original density and therewith de-laminated so as to form a plurality of not completely separated thin fibre layers that have a density corresponding to the first density.
Another conceivable acquisition layer is formed of a material layer having a first thickness and including resilient material, said layer being compressed perpendicularly to a plane through the layer to a second thickness and bound in its compressed state with a binder that is soluble in body liquid, wherein binding of the material layer ceases when the layer is wetted such that the acquisition layer will return at least partially to the first thickness. Such an acquisition layer may, for instance, be formed from a compressed foam material that will expand in its thickness direction when wetted, or by a compressed fibre layer which consists at least partially of fibres that have a given resiliency in a wet state.
According to another embodiment of the invention, that proportion of the volume of the acquisition layer that is comprised of the liquid-acquisition space is greatest within the primary wetting region of the article, i.e. the region in the article that is intended to be wetted first by body liquid. The acquisition layer volume shared by the liquid-acquisition space may therewith decrease in a direction away from the primary wetting region.
Because the volume of the liquid-acquisition space in the inventive article expands in keeping with the extent to which the article is wetted by body liquid, the article is able to maintain a high liquid acquisition rate during the full length of time in which the article is worn. Distinct from earlier known articles, there is no dramatic reduction in acquisition rate with repeated wetting occasions, since a new liquid-acquisition space is constantly created. In favourable cases, the inventive article can maintain essentially the same liquid acquisition rate after a plurality of wetting occasions. In particularly favourable cases, the acquisition rate may even increase after the first wetting occasion.
In order to achieve full effect of the expanding liquid-acquisition space in an inventive absorbent body, it is essential that at least the material layer that is arranged in abutment with the side of the storage layer that lies proximal to the liquid-permeable surface of the article has a resiliency and rigidity, both in a wet and a dry state, that will prevent said material layer from collapsing and falling into the liquid-acquisition space, since otherwise a large part of the space available for further liquid accommodation would be lost.
Those cavities, regions of low density, channels or the like that together form the it liquid-acquisition space of the article will preferably not have a dimension in the plane of the acquisition layer that exceeds 35 mm and preferably does not exceed 20 mm. By this is meant that the extension of each such cavity, region of low density, channel or the like will not accommodate within its area a circle having a diameter greater than 35 mm and preferably not greater than 20 mm anywhere in the plane of the acquisition layer. Because of the flexibility of the materials, it is difficult to avoid surrounding material layers from bulging in towards the liquid acquisition space with greater dimensions than those aforesaid, therewith reducing said space. Absorbent articles shape and curve in conformity with the wearer""s body in use. The material layers that lie proximal to the wearer therewith tend to curve and bulge into the acquisition space. This bulging generally increases when the material in the casing sheet is wet, and is even more pronounced when the storage layer includes large cavities. Naturally, this bulging can be reduced by using a stiffer material layer nearest the storage layer. However, the extent of the acceptable stiffness of such a material layer is determined by the requirements placed on the shapability, flexibility and user comfort of the article. The smallest functional dimension of cavities or the like in the storage layer correspond approximately to the size of a water droplet. Consequently, the size of a cavity or some corresponding space in the acquisition layer should not be so small as to enable a circle having a diameter of 3 mm or more to be enscribed inwardly of its boundary edges in the plane of the storage layer.